1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular, to a driving circuit of a white light emitting diode which configures a light source of a backlight.
2. Description of the Related Art
A small liquid crystal display device of a Thin Film Transistor (TFT) type in which a thin film transistor (TFT) is used as an active element has been widely used in a display unit of a mobile phone or the like. The liquid crystal display device includes a liquid crystal display panel, and a backlight which irradiates the liquid crystal display panel with light.
As a light source of the backlight, a white light emitting diode is used in the small liquid crystal display device. In the liquid crystal display device in which the white light emitting diode is used as the light source of the backlight, the white light emitting diode is driven with a constant current using a voltage which is stepped up in a main step-up voltage circuit, in an LED driving circuit which drives the white light emitting diode. An LED backlight driving circuit which drives an LED backlight is disclosed in JP2011-138666A.
In general, an LED driving circuit includes a main step-up voltage circuit which drives a white light emitting diode, and the main voltage step-up circuit includes a smoothing capacitor.
In addition, in a liquid crystal display device which uses a white light emitting diode as a light source of a backlight, in general, brightness of the backlight is adjusted by a PWM (Pulse Width Modulation) control which varies a duty ratio in periods of ON/OFF of the white light emitting diode.
When brightness of the backlight is adjusted by a PWM control, a current which flows in the white light emitting diode fluctuates between 0% and 100%. When a ceramic capacitor is used as the smoothing capacitor of the main voltage step-up circuit in a LED driving circuit, there is a problem in that the ceramic capacitor generates a sound since voltages at both ends of the ceramic capacitor fluctuate.
In order to solve the problem, as the smoothing capacitor of the main voltage step-up circuit, in the related art, an aluminum electrolytic capacitor, or a functional polymer capacitor has been used, instead of the ceramic capacitor.
However, in the aluminum electrolytic capacitor, or the functional polymer capacitor, there are problems such as (1) a life span is shorter than the ceramic capacitor, (2) internal resistances are high, (3) shapes of components are large, (4) a usable temperature range is small, and the like.